Polymorphisms in the repeat long regions of oncogenic and attenuated pathotypes of Marek's disease virus 1.

The nucleotide sequences of the terminal repeat long (TR(L)) and internal repeat long regions (IR(L)) in the genomes of 13 strains of Marek's disease virus type 1 (MDV-1) were determined and represent the largest collection of sequencing data from a contiguous region (12.8 kb) in the serotype 1 genomes. The collection of strains used in this study has been well characterized with respect to their virulence and contains members of each pathotype (4 attenuated, 1 mildly virulent, 3 virulent, 2 very virulent and 3 very virulent plus). It has previously been reported that two loci (meq and RLORF4) in the RL regions are likely to encode virulence factors based on comparative genomic studies involving vaccine and virulent strains. Additional studies using knockout mutants have provided stronger evidence that indeed RLORF4 and meq or the overlapping genes 23 kD and RLORF6 are involved in virulence. In this report, we provide evidence that additional open reading frames (ORFs) in the RL regions differ significantly between the extremes of the pathotypes (attenuated vs. nonattenuated). A deletion of 10 base pairs has been identified in RLORF12 from two attenuated strains CVI988 BP-5, p48 and RM-1, p40; and the lower virulence strain JM/102W. A deletion of 40 bp was also identified in RLORF4 of the attenuated strain R2/23, passage 106. A 177 bp insertion within the meq loci has been identified in most of the attenuated strains examined. Interestingly, R2/23 did not contain this insertion but instead truncated proteins are predicted for the three overlapping ORFs (meq, 23 kD and RLORF6) due to a frameshift mutation. Single nucleotide polymorphisms (SNPs), which loosely partition between attenuated and nonattenuated strains, have been identified in the ORFs encoding RLORF12, RLORF8, meq, 23 kD, RLORF6, RLORF4, RLORF3 and ICP0 and three previously unidentified short ORFs: MHLS, MLHG and MPSG. Although no single nucleotide polymorphism in the RL regions could predict virulence, their overall contribution to virulence can now be examined in defined mutants containing additional insertions or deletions in ORFs, suspected of encoding virulence factors, identified by this research.